This application deals with a general problem in virulence of how pathogenic microbes adapt to and/or escape host defenses and persist. Most infections are characterized by a complex relationship between the host and the pathogen referred to as pathoadaptation. The system under study is based on Cryptococcus neoformans (CN), a human pathogenic fungus that is a major cause of mortality and morbidity in AIDS patients. This fungal infection is notoriously difficult to treat and despite effective antifungal therapy mortality is high especially in AIDS patients with progressive disease. In the course of a R21 funded research proposal we determined that replicating CN cells undergo asymmetric cell divisions a process that leads to generational or replicative aging. CN cells undergo a finite number of divisions before they die. The replicative life span (RLS) of a CN strain is a reproducible trait that can be determined through micro-dissection. We found that CN cells of advanced replicative age (older) persist during chronic infection because they are more resistant to hydrogen peroxide stress, macrophage intracellular killing, and antifungal agents. We now propose to investigate the process of replicative aging in more detail and specifically elucidate how the emerging properties of aging lead to selection of older CN cells in the host, and contribute to persistence and virulence. Our proposal is divided in three aims: I.) To investigate RLS dynamics in CN strains ii.) To investigate why old CN cells are more resistant to antifungals and phagocytosis iii.) To investigate and model age dependent resilience and vulnerability in vitro and in vivo.